[procaryote]

Of these:

> * Fix bugs in third-party libraries without forking

> * Modify the behavior of existing functions

> * Add new features or options to existing classes

> * Test alternative implementations in an isolated way

only the last sounds close to something you might actually want to do, and then only as a throwaway thing

If you want to change a library, fork it. If you want to change the behavior of existing functions, don't or at least fork first. If you want to add new features to a class, write a new class, or again, at least fork first